Engineers are constantly seeking ways to improve both performance and efficiency in fuel injection systems. Performance improvements can lead to a reduction in undesirable emissions from the engines. Substantial improvements in performance have been achieved by providing fuel injectors with electronically controlled direct control needle valves. In general, a direct control needle valve includes a needle valve member with a closing hydraulic surface that can be exposed to either high pressure or low pressure, independent of engine speed and load. This innovation permits fuel to be injected at timings and in quantities that are electronically controlled independent of engine speed and load. This capability has allowed engineers to tailor engine operation to achieve certain goals, such as a reduction in undesirable emissions from the engine across its operating range. Although the implementation of electronically controlled direct control needle valves has allowed for improved performance, it has often come at the cost of a decrease in efficiency.
Efficiency relates generally to the amount of engine horsepower directed to powering the fuel injection system. One area in which efficiency problems can be revealed relates to the quantity of fluid pressurized by the fuel injection system which but leaked back for recirculation to a low pressure area. In other words, energy is arguably wasted whenever fluid, be it fuel or a hydraulic actuation fluid, is pressurized by an engine operated pump, but leaked back to tank without being used. For instance, in the case of common rail fuel injectors, two major static leakage sources exist, the needle guide and the needle push rod guide. During injector off time, both of these guides are exposed to injection rail pressure on one end with vent to tank pressure on the other end. Extreme measures are often employed to minimize the guide clearance(s) to reduce the static leakage. As the desired operating pressure levels are increased, the leakage problem becomes more and more severe. In addition, pressure induced deflections in the guide bores add to an already difficult situation. During injection, excessive leakage can sometimes occur through the needle control valve that controls the application of high or low pressure to the closing hydraulic surface of the direct control needle valve member. In some instances, the rail is connected directly to drain in order to perform the injection timing control function. While there are often flow restrictions positioned between the rail and the drain, substantial efficiency degradations can occur due to an excessive leakage of fuel back for recirculation in order to perform the control function. For instance, a fuel injection system that exhibits both these static and control leakage issues is described in “Heavy Duty Diesel Engines—The Potential of Injection Rate Shaping for Optimizing Emissions and Fuel Consumption”, presented by Messrs Bernd Mahr, Manfred Dürnholz, Wilhelm Polach, and Hermann Grieshaber, Robert Bosch GmbH, Stuttgart, Germany at the 21st International Engine Symposium, May 4-5, 2000, Vienna, Austria.
The present invention is directed problems associated with effectively combining performance and efficiency in fuel injection systems.